At present, there is a spring return actuator that uses a spring to forcibly operate a valve to a fully closing direction when interruption of electric service occurs. A spring return actuator has problems in that (I) power consumption is large because a motor for generating a large torque is necessary to obtain the load torque for driving a valve and the torque for winding a spring, (II) the target position during interruption of electric service can be set to only a fully closed position and cannot be set to a fully open position or another position, and (III) mechanisms, such as a clutch and a brake, for controlling operation during interruption of electric service are necessary, for example.
As means for solving these problems, there is proposed an electric actuator that returns the valve to a predetermined opening position by forcibly driving a motor during interruption of electric service using electric energy stored in an electricity storing portion, such as an electric double layer capacitor (see PTL 1).
Such an electric actuator eliminates the need to wind a spring using the rotating force of a motor. However, when a plurality of electric actuators 300-1 to 300-N are connected to one power supply system 301 as illustrated in FIG. 13, if an attempt is made to charge the electricity storing portions of the electric actuators 300-1 to 300-N all together when the electric power is turned on to reduce charge time, a problem occurs in that a relatively large charge current flows through the electric actuators 300-1 to 300-N, as illustrated in FIG. 14, so a power switchboard 302 needs to have a breaker with a large current capacity.
In addition, if an attempt is made to charge the electricity storing portions of the electric actuators 300-1 to 300-N in sequence after the electric power is turned on, a problem occurs in that the charge time until all of the electricity storing portions of the electric actuators 300-1 to 300-N have been charged becomes long, as is clear from FIG. 15 because charge time is relatively long.